1. Field of the Invention
The invention relates to one-component, silane-crosslinking compositions which are used as adhesives having high tensile shear strength and a high curing rate.
2. Description of the Related Art
Among the known means of implementing adhesive bonds on wood are wood glues, formulated typically on the basis of polyvinyl acetate dispersions. Although they exhibit good adhesion to wood, their setting rate, i.e., the time which elapses before a loadable bond is formed, is very long, and so mechanical fixing of the workpieces that are to be bonded, for some time, is generally unavoidable. Furthermore, the use of this type of adhesive presents problems if the bond is exposed to moisture, since the wood glues typically have only limited resistance toward water.
In the case of wooden constructions which are subject to high stresses, where the requirements imposed on the mechanical strength of the components are exacting and the bond strength is still to be high enough even after many years under the effects of weathering, wood glues of this kind are usually not appropriate.
Here, customarily, isocyanate-crosslinking PU adhesives are used. These adhesives typically comprise aromatic polyisocyanates. Systems of this kind cure by reaction of the isocyanate groups with (atmospheric) moisture.
Since PU adhesives cure via a chemical crosslinking reaction and are able to attach chemically as well to the wood substrate, they exhibit significantly better mechanical properties and are also substantially more resistant toward external (weathering) effects such as moisture or direct water contact.
The general performance of adhesives is specified through compliance with standards, such as, for example, DIN EN 204, durability classes D1-D4. These standards can generally be met by isocyanate-crosslinking adhesives.
Nevertheless, even some isocyanate-crosslinking adhesives possess massive disadvantages inherent in the system. For example, one-component PU adhesive systems generally possess no more than moderate cure rates. It is true that the isocyanate crosslinking can in principle be accelerated sharply by aggressive catalysis. However, since such catalysis in principle also catalyzes unwanted side reactions of the isocyanate groups (e.g., formation of allophanates, uretdiones, isocyanurates, etc.), the systems in question then no longer have sufficient shelf life.
Another disadvantage of the isocyanate-crosslinking adhesives is the sensitizing effect of all the isocyanate-containing compounds. Moreover, many monomeric isocyanates are toxic or even very toxic and/or are suspected of being carcinogenic. This presents problems insofar as the end user, i.e., the craftworker or do-it-yourself user, comes into contact not only with the cured and hence isocyanate-free and entirely unobjectionable product, but also with the isocyanate-containing adhesive. For the unpracticed home improver there is a particular risk here that the products may not be used expertly and/or properly. Additional hazards arise here from incorrect storage as well, such as storage within the reach of children. With the professional craftworker, on the other hand, proper use and storage can be assumed. Here, however, the problem may exist that the professional user is required regularly—possibly even a number of times a day—to work with the isocyanate-containing material, a fact which is potentially critical in view in particular of the aforementioned sensitizing effect of isocyanates.
Somewhat more favorable here are isocyanate-crosslinking adhesives which contain only very low levels of volatile isocyanates and are therefore at least free from labeling requirements. These adhesives, however, are mostly based on aliphatic isocyanates, which in turn are less reactive. For applications where rapid setting of the adhesive is a factor, therefore, these adhesives are once again less favorable than conventional PU adhesives.
Further disadvantages of isocyanate-based adhesives are, moreover, the tendency that occurs particularly in a humid environment (bonding at high atmospheric humidity and/or bonding of moist workpieces) toward blistering as a result of formation of CO2, and also the impossibility of removing unwanted adhesive contaminations without residue. The latter applies not only to cured but also to fresh residues of PU adhesive.
An alternative curing technology which is finding application increasingly in the adhesives sector is that of silane crosslinking, where alkoxy silane-functional prepolymers on contact with atmospheric moisture, initially undergo hydrolysis and then cure through a condensation reaction. The corresponding silane-functional—usually silane-terminated—prepolymers are entirely unobjectionable from the standpoint of toxicology.
While conventional silane-crosslinking systems have long had the disadvantage of a relatively low cure rate, more recent times have seen descriptions also of highly reactive systems, such as in EP 1414909 or in EP 1421129.
Customary silane-crosslinking adhesives have a backbone of long-chain polyethers having molar masses which are usually of the order of 10,000 daltons or more. Occasionally somewhat shorter-chain polyethers, typically with molar masses of 4000-8000 daltons, are used as well, and are then linked with diisocyanates to form longer units. Here as well, therefore, overall, very high molecular mass prepolymers are obtained, whose backbone continues to consist substantially of long-chain polyether units, the polyether chain being interrupted by a small number of urethane units. Systems of this kind are described in WO 05/000931, for example.
A disadvantage of all of these common silane-crosslinking systems, however, is a relatively low tensile shear strength. Consequently, customary applications for this new type of adhesive are generally confined to sectors in which the requirement is for elastic adhesives more than for adhesives of high tensile strength. Adhesives which meet European standard DIN EN 204, durability class D4, have not hitherto been achievable with silane-crosslinking adhesives.